The invention generally concerns control of fluid flow rates, and more particularly concerns the control of fluid flow rate in intravenous fluid delivery systems.
Intravenous (IV) fluid delivery systems are used to deliver fluids and medicines to patients at controlled rates. To more accurately control IV fluid delivery, an open-loop control system is typically used. A processor included in the open-loop control system varies the speed of a relatively accurate fluid pump used to infuse a medicinal fluid into a patient, based on a predefined algorithm and as a function of various parameters, such as temperature, fluid type, and desired flow rate. These open-loop processor-controlled pumping systems are generally expensive and complex. Usually, compensation for variations in pump accuracy must be employed in such systems to achieve an acceptable accuracy. The rate of fluid delivery is also affected by the precision of disposable components used in the fluid path that conveys a medicinal fluid to a patient. However, variations in the internal diameter and material hardness of fluid lines and pumping component comprising the disposable components, both initially, and as a result of changes over their period of use, cannot readily be compensated in an open-loop control algorithm. As a result, higher cost disposable components that are guaranteed to meet tight tolerance specifications must be used in such systems to avoid loss of accuracy.
Accordingly, it will be apparent that it would be desirable to provide a relatively low cost, low complexity system for delivery of medicinal fluids. A closed-loop system in which a desired parameter is measured to control the system can provide the required accuracy. For example, in a closed-loop system, it would be preferable to measure flow with a low cost flow sensor and to control an inexpensive fluid delivery pump based upon the measured flow rate, so as to achieve a desired flow rate. Previously, measurement of fluid flow has generally been prohibitively expensive in medicinal fluid infusion systems. However, the development of low cost flow sensors have made it much more practical and economical to monitor fluid flow in order to control a medical infusion system.
Low cost pumps can be used in a closed-loop system medicinal fluid infusion system, since the accuracy of the pump is not important in achieving a desired delivery rate. Similarly, the tolerance specifications for the disposable components used in the system can be greatly relaxed, because the precision of these components will no longer be of much concern. Also, most of the variables that must be considered in algorithms currently employed for open-loop control can be ignored in a closed-loop controlled infusion system. Consequently, the process control logic used in a closed-loop infusion system is relatively simple.
In accord with the present invention, a fluid delivery system is defined for infusing a medicinal fluid supplied from a reservoir into a patient at a desired rate. The fluid delivery system includes a fluid line through which the medicinal fluid is conveyed from the reservoir to a patient, and a flow controller that selectively varies a rate of flow of the medicinal fluid through the fluid line. A processor is controllably coupled to the flow controller and to a flow sensor that monitors a rate of flow of the medicinal fluid through the fluid line, producing an output signal that is indicative thereof. The processor responds to the output signal and operates the flow controller in a closed-loop process, to achieve the desired rate of infusion of the medicinal fluid into a patient.
In one preferred form of the invention, the flow sensor includes an orifice disposed in a fluid path through which the medicinal fluid flows in the fluid line, and the orifice has a cross-sectional size that is substantially less than that of the fluid line. A pressure-sensing module in the fluid line is configured to sense a pressure drop across the orifice, producing the signal indicative of flow rate. In one embodiment, the pressure sensing module includes a distal pressure sensor and a proximal pressure sensor, the distal pressure sensor being used for monitoring a distal pressure of the medicinal fluid, downstream of the orifice, and the proximal pressure sensor being used for monitoring a proximal pressure of the medicinal fluid, upstream of the orifice. A difference between the distal pressure and the proximal pressure signals is indicative of the rate of flow of the medicinal fluid through the fluid line.
In another embodiment, the pressure sensing module includes a differential pressure sensor that monitors a differential pressure across the orifice and in response thereto, produces the signal supplied to the processor, which is indicative of the rate of flow of medicinal fluid through the fluid line.
Preferably, the flow sensor is disposed in a xe2x80x9cYxe2x80x9d fitting in the fluid line. In one embodiment, the flow sensor is removably coupled to the processor through a connector. In another embodiment, the flow sensor is removably coupled to the processor.
In some cases, it will occasionally be desirable to provide a substantially greater flow of medicinal fluid that can be achieved through the orifice of the flow sensor, e.g., to prime the fluid line before connecting it to a patient. In this case, a bypass channel is provided within the fitting, generally in parallel with the orifice. The bypass channel is then selectively opened to enable the medicinal fluid to substantially bypass the orifice when a greater rate of flow of the medicinal fluid than the desired rate is required through the fluid line.
One preferred form of the invention employs a pump for the flow controller, and the pump forces the medicinal fluid through the fluid line and into a patient. Alternatively, an electronically controlled valve is employed for the flow controller, the medicinal fluid flowing through the fluid line under the force of gravity.
A user interface is preferably included to enable input by a user of the desired rate of medicinal fluid flow through the fluid line.
Another aspect of the present invention is directed to a method for controlling a rate of infusion of a medicinal fluid into a patient through a fluid path. The method includes steps that are generally consistent with the functions performed by the elements discussed above.